Among the conventionally-known throttle valves is one in the form of a disk-shaped butterfly valve mounted on a throttle shaft. FIG. 12 is a view explanatory of behavior of a typical example of the conventional throttle valve. Throttle shaft 202 is disposed in an intake air passage 201 perpendicularly across the longitudinal axis of the intake air passage 201, and a throttle valve 203 is mounted on the throttle shaft 202. As shown, an amount of air intake is adjustable by opening the throttle valve 203. So-called “wake”, comprising numerous vortex flows, occurs in airflows near the downstream side of the throttle valve 203, as indicated by arrows. However, the wake, occurring near the downstream side of the throttle valve 203, results in a great pressure drop and hence a great fluid resistance at the downstream side of the throttle valve 203. When the throttle valve 203 is opened, the great fluid resistance prevents a sufficient amount of air from being supplied to the combustion chamber, so that the number of rotations of the engine can not increase promptly. Thus, if the above-mentioned fluid resistance can be reduced in some way or other, the number of rotations of the engine may be allowed to increase more promptly, which may enhance throttle response.
Further, there have been known various examples of techniques for producing a swirl in the combustion chamber through arrangements of the air intake system having the throttle valve disposed therein, such as (1) the one employing a swirl control valve (see, for example, Japanese Patent Application Laid-open Publication No. HEI-11-247661), (2) the one employing a swirl control valve and a swirl port (see, for example, Japanese Patent Application Laid-open Publication No. 2002-235546), (3) the one employing a throttle valve disposed upstream of a pair of air intake ports and an inclined throttle shaft supporting the throttle valve (see, for example, Japanese Patent Application Laid-open Publication No. 2002-201968), and (4) the one employing a helical port (see, for example, Japanese Patent Application Laid-open Publication No. HEI-7-158459).
FIG. 13 is a diagram showing a general setup of an intake-air swirling current producing apparatus disclosed in the No. HEI-11-247661 publication mentioned in item (1) above, which is particularly explanatory of the conventional swirl producing technique employed therein. In an air intake tube 210, there are provided a throttle valve 211 and swirl control valve 212 disposed downstream of the throttle valve 211. Step motor 214, which is connected to a rotation shaft 213 of the swirl control valve 212, is controlled by a control unit 218 on the basis of output signals from a throttle opening (i.e., opening degree or opening position) sensor 215, intake air meter 216 and engine rotation speed meter 217, to open/close the swirl control valve 212 to thereby produce a swirl. However, because the technique shown in FIG. 13 requires the swirl control valve 212, shaft 213 and step motor 214 in order to produce the swirl and particular software in order for the control unit 218 to process the output signals from the throttle opening degree sensor 215, intake air meter 216 and engine rotation speed meter 217, the number of components of the air intake system would increase, which results in a complicated structure and increased overall size and cost of the air intake system.
FIG. 14 is a sectional view showing an intake-air swirling current producing apparatus disclosed in the No. 2002-235546 publication mentioned in item (2) above, which is particularly explanatory of the conventional swirl producing technique employed therein. In an air intake tube 220, there are provided a throttle valve 221, main and swirl ports 223 and 224 separated from each other via a partition wall 222 downstream of the throttle valve 221, swirl control valve 225 disposed within the main port 223, and a guide fin 226 for directing intake air flows within the main port 223 toward the swirl port 224. Reference numeral 227 represents a motor for driving the swirl control valve 225, 228 a controller for controlling the driving motor 227, and 229 a cylinder. However, in this case too, because the technique shown in FIG. 14 requires the partition wall 222 within the air intake tube 220, the swirl control valve 225 and guide fin 226 within the main port 223 in order to produce the swirl and the motor 227 and controller 228 in order to drive the swirl control valve 225, the number of components of the air intake system would increase, which results in a complicated structure and increased overall size and cost of the air intake system.
FIG. 15 is a view showing an air intake apparatus disclosed in the No. 2002-201968 publication mentioned in item (3) above, which is particularly explanatory of the conventional swirl producing technique employed therein. In the air intake apparatus, as seen in section (a) of FIG. 15, a throttle valve 233 is fixed via a throttle shaft 232 to a throttle body 231, a pair of air intake ports 237 and 238 communicate at one end with a downstream end of a throttle bore 236 defined by the throttle body 231 and communicate at the other end with a combustion chamber 241 through air intake valves 242. The throttle valve 233 comprises lower and upper valve members 244 and 245.
Further, in section (b) of FIG. 15, the throttle bore 236 has a concavely-curved surface 246 over its region that corresponds to a setting of an opening degree θ1 from a fully-closed position to an opened position of the valve member 245 for medium load operation. The concavely-curved surface 246 is formed into a shape corresponding to a trajectory of the outer periphery of the upper valve member 245. Thus, as the throttle shaft 232 is rotated slightly, the lower valve member 244 opens with the upper valve member 245 remaining closed, so that intake air is introduced only through the lower valve member 244 and then flows into the combustion chamber 241 through one of the air intake ports 237 (section (a) of FIG. 15) to produce a swirl in the combustion chamber 241.
As seen in section (c) of FIG. 15, the throttle shaft 232 is inclined at an angle θ2 relative to an axis line 247 interconnecting the respective centers of the air intake valves 242, and the axis line 247 is offset from the center of the throttle bore 236.
With the technique of FIG. 15, only one of the air intake ports 237 and 238 may be provided in order to simplify the structure of the air intake system. However, in such a case, the outlet of the air intake port 237 or 238 has to be disposed at a predetermined position offset from the center of the combustion chamber 241, and thus, the design freedom of the air intake system would be significantly limited.
Furthermore, FIG. 16 is a view showing an air intake apparatus disclosed in the No. HEI-7-158459 publication mentioned in item (4) above, which is particularly explanatory of the conventional swirl producing technique employed therein. In the air intake apparatus, first and second helical ports 251 and 252 communicate with each other in a cylinder to produce a swirl A. Reference numeral 253 represents a first air intake valve for opening/closing the first helical port 251, and 254 represents a second air intake valve for opening/closing the second helical port 252. However, with the technique of FIG. 16, the first and second helical ports 251 and 252, each having to have a complicated shape, can not be formed easily. Besides, the first and second helical ports 251 and 252 each have to have a sufficiently-long port length, which tends to be disadvantageous in terms of productivity, cost, weight and air pressure.